The present invention generally relates to the preparation of semiconductor grade single crystal silicon, which is used in the manufacture of electronic components. More particularly, the present invention relates to a process for the controlled arsenic doping of single crystal silicon, prepared in accordance with the Czochralski method, in order to achieve low resistivity therein.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon, and a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by decreasing the pulling rate and/or the temperature field in the melt until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter may be reduced gradually to form a conical opposite end. Typically, the opposite end is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
Arsenic is a desirable dopant for single crystal silicon grown by the Czochralski method because, due to the high solubility of arsenic in silicon (e.g., about 4% by weight, or about 2×1021 atoms per cm3, at about 1100° C.), low resistivities can generally be achieved. Conventionally, the silicon melt is doped by feeding arsenic onto the melt surface from a feed hopper located a few feet above the silicon melt level. However, this approach is not favorable because arsenic is highly volatile and readily vaporizes at temperatures higher than 617° C. Thus, when the arsenic contacts the silicon melt surface, which is at a temperature of about 1400° C., it immediately vaporizes and is lost to the gaseous environment in the crystal puller. Vaporization loss of arsenic vapors to the surrounding environment typically results in the generation of oxide particles (i.e., sub-oxides). These particles can fall into the melt and become incorporated into the growing crystal, which is unfavorable because they can act as heterogeneous nucleation sites and ultimately result in failure of the crystal pulling process (due to a loss of zero-dislocation crystal growth).
The sublimation of arsenic granules at the melt surface often causes a local temperature reduction of the surrounding silicon melt, which in turn results in the formation of “silicon boats” around/beneath the arsenic granules; that is, arsenic sublimation at the melt surface results in localized freezing of the melt surface, causing the formation of solid silicon particles which act as “boats,” helping the arsenic granules to float on the melt surface. These silicon boats, along with the surface tension of the melt, thus prevent many of the arsenic granules that do reach the melt surface from sinking into the melt, thus increasing the time during which sublimation to the atmosphere can occur. The inability of the arsenic granules to sink into the melt therefore results in a significant loss of arsenic to the gaseous environment and further increases the concentration of contaminant particles in the growth chamber. In fact, typically only about 40% of the arsenic fed into a crystal pulling apparatus actually dissolves into the silicon melt before being lost to the environment. Thus, an extremely large amount of arsenic must be fed into a crystal pulling apparatus especially when attempting grown a low resistivity crystal.
In view of the foregoing, it can be seen that a need continues to exist in the semiconductor industry for a simple, cost effective approach to produce low resistivity, arsenic-doped single crystal silicon by the Czochralski method.